1. Field of the Invention
This invention relates to a continuous jet type ink jet recording apparatus, and more particularly to a technique for controlling the recording dot position of a continuous jet type ink jet recording apparatus accurately to improve the picture quality.
2. Description of the Related Art
An apparatus wherein the number of ink drops to be hit upon a single pixel is variably controlled using an ink jet recording technique of the continuous jet type to vary the recording dot diameter or dot size to represent a concentration is already known and disclosed, for example, in U.S. Pat. No. 4,620,196 or Japanese Patent Laid-Open Application No. Showa 62-225363.
Referring to FIG. 10, there is shown in diagrammatic view an exemplary one of a conventional continuous jet type ink jet recording apparatus of the rotary drum type. The continuous jet type ink jet recording recording apparatus shown includes, as principal components thereof, a nozzle 1 to which ink under pressure is supplied, an ink electrode 2 for connecting the potential of the ink in the nozzle 1 to the ground potential level, a vibrating element 3 mounted on the nozzle 1, an oscillator OSC for generating a disintegrating frequency signal f.sub.d having a fixed disintegrating frequency f.sub.d (in the following description, a same reference character is applied to both of a signal and a frequency), a vibrating element driver CD for amplifying the disintegrating frequency signal f.sub.d from the oscillator OSC to drive the vibrating element 3 and synchronously disintegrate a jet of the ink, a control electrode 4 having a circular opening or a slit-like opening coaxial with the nozzle 1 for receiving a charge control signal .phi..sub.C to control charging of the ink jet in accordance with pixel data (pixel density data) D.sub.P, a grounding electrode 5 disposed in front of the control electrode 4 and grounded itself, a knife edge 6 mounted on the grounding electrode 5, a deflecting high voltage dc power supply (hereinafter referred to simply as deflecting power supply) 7, a deflecting electrode 8 connected to the deflecting power supply 7 for cooperating with the grounding electrode 5 to produce therebetween an intense electric field perpendicular to an ink jet flying axis to deflect a charged ink drop to the grounding electrode 5 side, a line buffer LB for storing therein pixel data D.sub.P for one rotation of a rotary drum DR for generating the charge control signal .phi..sub.C, a pulse width modulator PWM for modulating pixel data D.sub.P read out from the line buffer LB in synchronism with an encoder clock (dot recording clock) signal f.sub.E from a shaft encoder SE coupled to a shaft of the rotary drum DR into a width of a pulse in synchronism with the encoder clock signal f.sub.E and the disintegrating frequency signal f.sub.d outputted from the oscillator OSC, and a high voltage switch HVS for converting a charge control signal S.sub.C outputted from the pulse width modulator PWM into a high voltage charge control signal .phi..sub.C. It is to be noted that, in FIG. 10, reference symbol RM denotes a recording medium wrapped around the rotary drum DR. Further, reference symbol O.sub.P denotes an origin pulse signal which provides a timing at which the recording starting position (origin) of a main scanning line in a circumferential direction of the rotary drum DR is to be indicated.
The pulse width modulator PWM converts pixel data D.sub.P read out from the line buffer LB into a charge control signal S.sub.C having a pulse width corresponding to the value of the pixel data D.sub.P. The pulse width modulator PWM is formed from, for example, a presettable counter. In particular, if the preset counter is preset with the preset data D.sub.P in response to the encoder clock signal f.sub.E and the disintegration frequency signal f.sub.d is inputted as a down clock signal to the pulse width modulator PWM, then the time until the preset down counter becomes empty after the presetting point of time of the preset down counter provides the pulse width of the charge control signal S.sub.C.
FIG. 11 illustrates in diagrammatic view a principle wherein the dot size is variably controlled by pulse width modulation which is used in the continuous jet type ink jet recording apparatus shown in FIG. 10. Here, for convenience of illustration, it is shown that nine gradations are represented and the recording apparatus is designed such that the encoder clock signal f.sub.E which is an output of the shaft encoder SE has a frequency equal to one eighth the frequency of the disintegrating frequency f.sub.d outputted from the oscillator OSC and is locked in phase with the disintegrating frequency signal f.sub.d. Eight ink drops in one period of the encoder clock signal f.sub.E forms one pixel. While the dot size is controlled depending upon the number of ink drops from among the eight ink drops per period of the encoder clock signal f.sub.E should be made of non-charged ink drops, the non-charged ink drop number is stored as pixel data D.sub.P in the line buffer LB. In FIG. 11, .circle-solid. denotes a non-charged ink drop, which advances straightforwardly without being deflected and is recorded on the recording medium RM, and .largecircle. denotes a charged ink drop, which is deflected and cut by the knife edge 6 and consequently does not reach the recording medium RM. Particularly, FIG. 11 illustrates the formation of a first pixel with one ink drop, a second pixel with three ink drops, and a third pixel with five ink drops.
In the conventional continuous jet type ink jet recording apparatus having the construction described above, a non-charged ink drop train to be recorded flies in the air and is decelerated by the air resistance. FIG. 12 is a diagrammatic view illustrating a behavior in which an ink drop train to for forming a pixel flies in the air. Now, it is assumed that five example ink jets which are equal in jet flying speed, disintegrating frequency fd and particle size are prepared and charge control signals S.sub.C (.phi..sub.C) with which the number of non-charged ink drops per pixel is 1, 2, 3, 4 and 5 are applied simultaneously to the control electrode 4 (position "A" in FIG. 12). If the ink dot trains enter the deflecting electrode 8, then charged ink drops begin to be deflected downwardly of the jet flying axes and into the knife edge 6 by an action of the deflecting electric field ("B"). As the ink dot trains further advance in the deflecting electric field, since, in each of non-charged ink drop trains on the jet flying axes, the leading or forwardmost ink drop is acted upon by the highest air resistance, the following ink drops are gradually and successively integrated with the leading or forwardmost ink drop ("C"). With the integrated ink drop, the rate of the increasing amount of the inertial force (which increases in proportion to the third power of the particle size) becomes larger than that of the increasing amount of the air resistance (which increases in proportion to the second power of the particle size), and the degree of deceleration by the air resistance decreases. As a result, after drop integration starts, a non-charged ink drop train which has a smaller number of ink drops per pixel exhibits a larger delay, and when it passes by the knife edge 6 and arrives at the recording medium RM on the rotary drum DR, such a delay as seen in FIG. 12 is produced ("D"). By this delay, a dot of a smaller size (a dot having a lower pixel density) is recorded with a larger delay in a direction opposite to the direction of rotation (main scanning direction) of the rotary drum DR, and a positional displacement of the recorded dot corresponding to the dot size is produced.
In order to solve the problem described above, the inventor of the present invention has already proposed an ink jet recording apparatus of the continuous jet type wherein the application timing of a charge control signal S.sub.C (.phi..sub.C) is delayed in response to the dot size (the delay time of a dot having a larger size is set longer) to correct the positional displacement of a recorded dot (refer to Japanese Patent Laid-Open Application No. Heisei 5-246034). While the problem mentioned above has been solved by the ink jet recording apparatus of the continuous jet type just mentioned, a new problem that the recording time is increased has arisen. In particular, with the ink jet recording apparatus of the continuous jet type mentioned, since the delay time for a larger dot size (larger number of non-charged ink drops) must be set longer, also those ink drops which are included in the delay time must be necessarily included in the number of ink drops per pixel but are nonetheless wasted. For example, while, in the case illustrated in FIG. 11, eight ink drops in one period of the encoder clock signal f.sub.E form one pixel, where the application timing of the charge control signal S.sub.C (.phi..sub.C) is delayed in response to the dot size, in order to represent the same nine gradations, approximately 12 ink drops must be allocated to one period of the encoder clock signal f.sub.E. As the number of ink drops per one pixel increases. The running cost is increased because of ink drops associated with the delay by wasteful ink and an increase in recording time.
Further, the inventor of the present invention has proposed another ink jet recording apparatus of the continuous jet type wherein, in order to solve the problems of an increase in running cost and an increase in recording time, correction charge corresponding to a dot size is provided to each ink drop to be recorded (hereinafter referred to as recording ink drop) and the jet flying axis of the recording ink drops is displaced in units of a pixel toward a deflection electrode side, whereby a recording dot can be positioned accurately irrespective of the dot size (refer to Japanese Patent Laid-Open Application No. Heisei 7-290704). While the two problems described above have been solved by the apparatus just described, a different problem arises in that a circuit system necessary to control the correction charge amount at a high speed is complicated and the cost for hardware increases and another different problem that some deterioration in picture quality arising from the fact that a recording ink drop has charge and the flying axis of a recording ink drop varies depending upon the dot size.